1. Field of the Disclosure
The present disclosure relates generally to a supply modulator (SM) used for radio transmitters in communication systems.
2. Description of Related Art
Battery power consumption is considered to be a very important aspect of applications for radio communication terminals. Thus, a highly efficient radio frequency (RF)/analog output amplifier is required for long battery use. With the evolution from 2G to 3G/4G, highly efficient characteristics are required with a high peak-to-average power ratio (PAPR). Furthermore, 4G communication channels demand terminals having hardware for high linearity and high bandwidth. The RF/analog output amplifier required in terminals for RF communication has procedural and circuit-structural limitations that make it difficult to satisfy all of the above-described requirements.
The efficiency of an RF power amplifier (RF PA) decreases as the PAPR of an input signal increases. Since high PAPR characteristics require a high 1 dB gain compression point (P1 dB) and saturated power of the RF PA, all existing RF PAs operating with limited fixed supplies, such as, for example, batteries, have low power efficiency in peak power and back-off power regions.
Therefore, to improve the low power efficiency in the back-off power region, an average power tracking (APT) technology has been developed to control a fixed supply voltage while tracking an average power. However, when the APT technology is applied, the fixed supply voltage is unable to instantaneously catch up with an envelope signal, and thus, the RF PA suffers additional power loss.
In an attempt to rectify the additional power loss, an envelope tracking (ET) technology has been developed. The ET technology increases the RF PA efficiency, improving the efficiency of the RF PA by instantaneously following an input envelope signal. Accordingly, to apply the ET technology to the RF PA, a supply modulator (SM) is required to normalize battery power for an envelope signal. The SM is required to have both a high bandwidth and a high efficiency, and thus, typically employs a hybrid structure in which a linear regulator including a linear amplifier and a switching regulator are combined.
FIG. 1 is a block diagram illustrating a transmitter including a general hybrid SM.
Referring to FIG. 1, a modem 110 provides an RF integrated circuit (RFIC) 120 with a transmit signal containing information for transmission. The modem 110 also provides an SM 140 with an envelope signal generated by modulating the transmit signal.
The RFIC 120 outputs an RF signal by modulating the transmit signal into carrier waves of the system band, and an RF PA 130 amplifies the RF signal to a required power level and transmits the amplified RF signal through an antenna.
The SM 140 delivers battery power to the RF PA 130 in the procedure of amplifying and normalizing the envelope signal input from the modem 110. The SM 140 controls the fixed supply power provided from the battery based on the envelope signal, so that the SM 140 and the RF PA 130 may have optimum linearity and efficiency.
Furthermore, the SM 140 of FIG. 1 has a hybrid structure in which a switching regulator 142, and a linear regulator 141 including a linear amplifier are combined. Outputs of the linear regulator 141 and the switching regulator 142 are combined by a combiner, and the combined result is provided to the RF PA 130 as a supply voltage of the RF PA 130. The linear regulator 141 guarantees high output accuracy in the supply voltage of the RF PA 130 by tracking the high frequency region of the envelope signal. The switching regulator 142 tracks a low frequency region of the envelope signal to provide a wide range of output voltage.
The RF PA 130 tracks the saturated power and amplifies an output signal of the RFIC 120 with the power supplied from the SM 140. In this regard, the RF PA 130 operates in an average power tracking (APT) mode or an envelope tracking (ET) mode, depending on the output power. Specifically, if the output power is less than a predetermined value, the RF PA 130 operates in the APT mode and uses an output of a buck-boost converter within the SM 140 as the fixed power. On the contrary, if the output power is greater than or equal to the predetermined value, the RF PA 130 operates in the ET mode and uses an output of the SM 140 of the hybrid structure as the fixed power.
The efficiency of a power amplifier is determined by the product of an efficiency of the SM and an efficiency of the power amplifier itself. Since the efficiency of the SM decreases and the ET effect is reduced as the output power is backed off, the efficiency of the power amplifier in the APT mode becomes higher than the efficiency of the power amplifier in the ET mode below a certain value (back-off power). Accordingly, to widen the dynamic range of the ET mode, both the ET effect and the back-off power efficiency of the SM should be improved.
Furthermore, to maximize the efficiency of the power amplifier in the ET mode, the SM tracks a saturated voltage by controlling the supply voltage of the RF PA. In this regard, due to the tracking of the saturated voltage and a knee voltage effect, additional shaping of an output voltage of the SM is required. A shape table that is optimized for back-off power values is required to improve the back-off effect of the power amplifier in the ET mode by adaptively controlling the output voltage of the SM based on the back-off power. However, output voltage shaping by power control requires an increase in the complexity of the operation of the communication system and an RF driver.
Furthermore, the output voltage of the hybrid SM is scaled down in proportion to the back-off power. In other words, there is an efficiency loss of a linear amplifier in proportion to a ratio of output voltage to fixed supply voltage, which reduces the entire efficiency of the SM due to the back-off power. Since the maximum fixed supply voltage of the linear amplifier is determined by a peak output voltage, the fixed supply voltage of the linear amplifier, which is high in the back-off power region, affects the entire efficiency of the SM.